1. Field of the Invention
The present invention relates to a control circuit that controls the actuation of means for pressurizing or depressurizing a fluid-pressurized shock absorber in response to a sensor signal indicating the need for a change in pressure.
2. Description of the Prior Art
Load-leveling devices are used on vehicles subjected to varying load conditions to maintain a permissible body-to-axle distance for the vehicle. A common form of load-leveling device is an air-spring shock absorber which uses a pressurized air chamber to provide a spring-like effect. The pressure within the air chamber may be increased or decreased to adapt to varying load conditions. Generally, the air chamber is provided with an inlet valve and outlet valve to allow for the admission of air, such as by a pump, and release of air, such as by an exhaust valve, to and from the air chamber. The pressurized chamber is commonly provided with a sensor or detector that monitors the loading of the shock absorber with respect to a permissible range of body-to-axle distances. The sensor will provide a first signal condition when the shock absorber is overloaded, a second signal condition when under-loaded, and a third signal condition when properly loaded. Control circuits of various designs are known in the prior art to control the operation of the pressurization means and depressurization means in response to the various signal conditions from the detector. More specifically, the circuits respond to an overload or underload signal condition to turn on the pump or the exhaust until the axle-to-body distance is brought back within a permissible range. A control system of this type is disclosed in the Elliott U.S. Pat. No. 3,575,442.
It is also known in the prior art to provide a control system which discriminates between static and dynamic loading effects. More specifically, control circuits are provided with time delays so as to respond only to changes in static loading of the vehicle, and not to changes related to transient, dynamic loading effects. Examples of the latter are when the vehicle attitude changes through coming to a sudden stop or cornering around a long bend. Control circuits of this type are shown in the Boyer U.S. Pat. No. 3,633,935, Ciolli U.S. Pat. No. 3,727,941 and the Joneleit U.S. Pat. No. 3,830,138. Moreover, it is known to provide a control circuit which is reset so that successive transitory signals do not cause a signal buildup and a spurious pressurization or depressurization of the air chamber. Such a circuit is shown in Joneleit U.S. Pat. No. 3,873,123.
The control circuits of the prior art which are adapted to discriminate between steady-state and transitory load changes are of complicated and costly design. For example, the Joneleit U.S. Pat. No. 3,873,123 discloses a circuit which is implemented in discrete semi-conductor devices and electromechanical relays. The Joneleit U.S. Pat. No. 3,830,138 describes a system which uses a combination of integrated circuit components and discrete components along with several electromechanical relays. In particular, this circuit uses a large number of NAND gates and inverters in a logic arrangement and uses discrete components for a timing circuit. In this logic arrangement, loading of the vehicle and closure of the sensing switches results in a logic state which starts the pump and raises the vehicle. The logic state is latched on so that the vehicle continues to be raised until the upper limit switches are closed which results in a logic state which opens the exhaust valves and the vehicle is lowered until the upper limit switches are reopened. The adjustment cycle for the case of unloading the vehicle is the reverse of that described. Accordingly, the logic is complicated and a large number of gates are required.